This invention relates to epitaxial growth processes and, more particularly, to liquid phase epitaxy (LPE) using boat-slider apparatus.
In the prior art, horizontal growth of Group III-V compounds is commonly accomplished using a boat-slider apparatus and ramp-cooling techniques. U.S. Pat. No. 3,741,825 granted to H. F. Lockwood et al on June 16, 1973 is illustrative of both the apparatus and technique. Briefly, the apparatus comprises a graphite solution holder or boat having a plurality of tandem wells for carrying source solutions (melts). A graphite slider having at least two recesses in tandem, one for carrying a substrate and another for carrying a saturation seed, is inserted in a channel which extends horizontally through the boat and beneath the wells. The saturation seed precedes the substrate under each solution. Thus, in operation the boat is loaded with appropriate source solutions (e.g., Ga solutions of GaAs), and the slider is loaded with the substrate and saturation seed. The apparatus is then placed in a quartz tube within a furnace. After flushing of the ambient with a suitable gas such as hydrogen, the furnace temperature is raised to a temperature at which the source solutions are saturated (about 800.degree. C for GaAs). A controlled cooling program is then instituted and the slider is moved until the saturation seed is located under the first well to establish local liquidus equilibrium. Then the slider is again moved until the saturation seed is under the second well and the substrate is under the first well. Epitaxial growth takes place on the substrate at a rate determined by the cooling rate. Simultaneously the saturation seed establishes local liquidus equilibrium at the bottom of the second solution. Repetition of these steps results in the growth of multilayered structures, for example, GaAs-AlGaAs double heterostructure junction lasers.
During epitaxial growth by this equilibrium cooling technique, when the furnace temperature is lowered, heat flows outward from the melt, giving rise to both horizontal and vertical temperature gradients. These gradients produce a nonuniform temperature profile near the melt-substrate interface, with consequent nonuniform epitaxial growth. These nonuniformities exhibit themselves in at least two ways: variations in layer thickness across the usable, central portion of a wafer and excessive edge growth along the periphery of a wafer. Layer thickness variations result in obvious problems of device reproducibility. Edge growth reduces usable wafer area and, depending on boat-slider spacing, may scratch particles of graphite onto the wafer, disrupting epitaxial growth of subsequent layers.